1. Field of the Invention
This invention relates to a multistage continuous rolling mill, and more particularly to a system for controlling the re-distribution of a load in a longitudinal direction of a single strip (which is called hereinafter within the plate) on a continuous hot rolling mill. More specifically, it concerns a re-distribution-of-load control apparatus for maintaining a predetermined ratio of the distribution of a rolling force to a multistage continuous rolling mill comprising an automatic screw-down setting device and a master drive control for the rolling mill and for preventing the shape of a product and more particularly the flatness thereof from deteriorating within the plate or suppressing an inclination of a rolling load toward a specified rolling mill.
2. Description of the Prior Art
The distribution of a load (a load called herein implies a rolling force) to each of a plurality of roll stands on a continuous rolling mill is an extremely important subject in view of the standpoint of the ensuring of a product's shape and the maintenance of the smooth operation thereof. In conventional continuous rolling mills, for example, continuous hot rolling mills, therefore, the distribution of a load to each stand which has been predetermined through an initial setting calculation (the setting before the metal-in-stand) so that it is preliminarily of a proper ratio but the exact control has not been effected with respect to the monitoring and correcting of the distribution of the rolling force in a longitudinal direction of a material after the rolling of the material has been initiated, which is called the passage of the plate.
On the other hand, the material rolling conditions are momentaly changed within the plate of a material for both the main causes on the side of the material and those on the side of the rolling mill. As a result, it is natural that the distribution of the load to each rolling mill is also varied upon and after the initial setting.
This situation is described by taking the case of a conventional hot finish rolling mill as shown in FIG. 1. In FIG. 1, element 1 designates a working roll on the hot finish rolling mill; element 2 is a backup roll; element 3 is an automatic roll-opening positioning device; element 4 is a main control system for the driving speed of the rolling mill; element 5 is a looper which is located between stands; element 6 is a looper height control system, element 7 is a rolling force sensor (a load cell); element 8 is an automatic gauge control device (which is called an RF.multidot.AGC); element 9 is a monitor AGC device; element 10 is a high speed X-ray AGC device; element 11 is a product gauge sensor disposed adjacent to the exit side of the finish rolling mill and S designates a rolled material (i.e. a strip). In FIG. 1, the strip S is successively gripped by stands of from F.sub.1 to F.sub.7 and as a result, a preliminarily estimated rolling force Pi is generated on the load cell 7 on each stand. When the strip S is gripped by each stand, the RF.multidot.AGC provided on each stand is actuated so as to tend to maintain an exit gauge on each stand at a stored value (which is called a lock-on value) at the beginning of a passage of the plate. Also, upon the strip S reaching the product gauge sensor 11, the monitor AGC device 9 and the high speed X-ray AGC device 10 are further actuated to control the final product's gauge to be held at a predetermined absolute gauge. Also, in order to maintain the tension between the stands at a constant value and to render the mass flow between the stands at a constant value in the steady state, the looper 5 is present and the looper height control system 6 effects the fine adjustment of each speed control system 4 for the rolling mill.
While the hot finish rolling proceeds as described above, the material on the entry side of the finishing stand has a temperature drop due to the heat dissipation, for example, and, as the trailing end of the plate is reached, the temperature member lowers in temperature. On the other hand, the temperature of the plate adjacent to the last stand is held substantially constant as a result of the temperature control of the material on the finishing exit side, which control being effected by the acceleration of the rolling mill, the flooding between the rolling mills, etc. This results in a rise in the plastic coefficient of the material on the first half of the stands and also results in the rolling forces becoming large. FIG. 2 is an example of the actual measurement conducted with respect to the temperatures of a random sampled member on the entry and exit sides of a finishing stand. A temperature difference of about 100.degree. C. exists between the leading and trailing ends of the material on the entry side and an increase in rolling force of about 400 tons at the F.sub.1 mill results in an increase of about 20% from the rolling force at the beginning of a metal-in-stand. On the other hand, the range of variations in the exit temperature is about 20.degree. C. and consequently, a variation in the rolling force is about 10%.
Considering also the AGC operation, the RF.multidot.AGC, for example, does not take account the mutual load balance among the stands because the gauge control is independently effected for each stand. FIG. 3 is a block diagram illustrating the principles of the RF.multidot.AGC wherein 31 is the characteristics of the rolling mill; 32 is a mill elongation percentage (the reciprocal of a mill elastic constant); 33 is a tuning percentage; 34 is a lock-on value memory; 35 is a gain (a coefficient of influence); and 36 is the modeled characteristics of an automatic screw-down position setting device for the rolling mill. In this system it is well known that the construction is such that the main body of the rolling mill is regarded as an elastic body, and a screw-down position (S) is thereby so as corrected to compensate for an elongation (P/M) of a mill housing due to a rolling force, and the exit gauge on the rolling mill is maintained constant. In this system, .alpha. of the element 33 in FIG. 3 is a positive constant called the tuning percentage and since it is approximately equal to 1, the ability to fully absorb the mill elongation so as to maintain the gauge on the last exit side constant becomes high. However, if .alpha. is selected to be 1, then a rise in the rolling force which is due to, for example, the abovementioned temperature fall of the plate on the entry side, results in a further increase in the rolling force. (If the rolling force increases in the RF.multidot.AGC, then an screw-down opening is controlled so as to be correspondingly small so that the rolling force is further increased.) With the distribution of the rolling force in view, it is normally used as .alpha.&lt;1. That is, with the RF.multidot.AGC operated, the distribution of the rolling force among the individual stands is varied by selecting the tuning percentage .alpha. thereof. Further considering the feedback control from the last exit gauge sensor, the stands in the later stage are controlled by putting relative importance on the high speed response characteristic because a gauge deviation is absorbed at a high speed whilst the first half of the stands does not increase in control gain and is slowly controlled because there is a time delay for sensing a signal (i.e. a transportation delay). Therefore, if an initial setting calculation, for example, has a large error, then the second half of the stands is apt to have their rolling forces increase and furthermore, a ratio of the distribution thereof is dependent upon the selection of a feedback gain with the result that the rolling force on each stand is varied with respect to a value of the feedback gain. Assuming, for example, that the feedback has been concentrated on the last stand F.sub.7, and assuming that the setting calculation has an error of 200.mu. (0.2 mm), then if the mill constant is 600 tons/mm and the plastic coefficient is 500 tons/mm, then a rise in the rolling force on F.sub.7 is about 0.2.times.(600+500)=220 tons which is not permissible.
With the progress of the rolling, the distribution of the load to each rolling mill of the continuous rolling mill is gradually varied in this way within the plate and if such a variation in distribution of the load is left as it is, then this has given rise to the deterioration of the shape (i.e. flatness) of the final product or the concentration of the load on a specified stand so as to deteriorate the quality of the product and, in addition, has been the main cause for preventing the rolling mill from increasing its operational efficiency.